Motor circuit breakers protect electric motors by thermally tripped shutoff prior to overloading. They are generally combined with electromagnetic fast trippers, and intended primarily for three-phase rotary-current motors.
Motor circuit breakers can have a setting range for setting the nominal current of the electric motor, the nominal current range extending from a minimum settable nominal current Inenn,min to a maximum settable nominal current Inenn,max, and the nominal current Inenn that is set being within this nominal-current range.
Thermal protection is generally effected via bimetals that are heated by heating coils through which the motor current flows. If the power consumption of even one winding of the motor exceeds the predetermined value for several seconds, the bimetal becomes deformed by the heat and trips a trip unit of the motor circuit breaker, and interrupts the power circuit to the motor.
The electromagnetic fast tripper, on the other hand, causes an immediate shutoff in the event of a short circuit in the motor supply lead or a winding fault; typically, the electromagnetic fast tripper responds, and performs the fast trip, only above 14 times the maximum settable nominal current.
The use of bimetals in motor circuit breakers has the disadvantage that the setting range of the nominal current of the motor circuit breaker is greatly limited. The setting range of the nominal current can be expanded by replacing the bimetals with an electronic system in which the currents delivered to the motor are sensed by a current transformer, and shutoff of the motor takes place as a function of the sensed currents.
The current transformer is, as a rule, a transformer whose primary winding has passing through it an alternating current delivered to the electric motor, and in whose secondary circuit the transferred current is sensed. This transformer principle has the disadvantage, however, that the transformer core goes into saturation when the primary current exceeds a certain value. Current transformers typically go into saturation above about 8 times the maximum settable nominal current Inenn,max. Current transformers that go into saturation only at higher currents are significantly more cost-intensive and are physically larger. Saturation changes the sinusoidal secondary-side current signal, and spikes occur in the current signal, so that an effective-value calculation of the current using an A/D converter is possible only at a very high sampling rate.
In the region from 8 to 14 times the maximum settable nominal current Inenn,max, the motor circuit breaker can thus, longer sense the current delivered to the motor, and trip the motor, using an A/D converter having a low sampling rate. The sampling rate of the A/D converter would need to be greatly increased for correct current sensing in this region. This entails the disadvantage that this A/D converter having an elevated sampling rate requires more outlay for implementation, and thus causes higher costs and has a physically larger size. A further disadvantage is the fact that the higher sampling rate of the A/D converter causes higher dissipated power consumption.